1. Field of the Invention
This invention relates generally to a bulk substance detection systems for detecting concealed explosives and narcotics employing either nuclear quadrupole resonance or nuclear magnetic resonance, and more particularly to a practical system and method for Q-damping the detection receiver of a contraband detection system thereby improving operational capabilities.
2. Discussion of the Related Art
Certain atomic nuclei, typically having a spin quantum number of 1/2, exhibit magnetic signatures when they are within an externally applied magnetic field. This magnetic resonance effect is most commonly observed in .sup.1 H, and is known as nuclear magnetic resonance (NMR). Atomic nuclei with a spin quantum number of &gt;1/2 can also show another magnetic signature associated with the interaction of the nuclei with the local electric field. This phenomenon is known as nuclear quadrupole resonance (NQR).
For both of these phenomena, the energy level transitions are observed primarily in the radio frequency range. Detection of these transitions thus requires a radio frequency source to excite the transition, and a radio frequency receiving mechanism to detect the signal. Normally, the signals appear at a pre-defined frequency. An RF coil "tuned" to, or close to, that predefined frequency can excite or detect those signals. The signals are of very low intensity and can only be observed for a short time, approximately 10 .mu.s to 2 ms. As a consequence, there is a need for an NQR or NMR detection receiver that can be tuned to (usually) high Q, has very low noise, and is capable of fast recovery after a high voltage RF pulse. In most conventional resonance (NMR and NQR) experiments, small and fairly homogeneous samples are investigated.
Over the past few years there has been considerable interest in the larger-scale "real world" applications of both NQR and NMR. These applications do not benefit from the luxury of small-scale laboratory investigations. They usually require investigation of large volumes filled with materials of vastly differing physical and chemical composition. Investigation of the contents of mail or baggage for the presence of explosives or narcotics is an example.
With respect to explosives, plastic explosives, have an almost infinite variety of possible shapes and uses for terrorist bombing tactics. Plastic explosives are highly stable, have clay-like malleability and are deadly in relatively small quantities. A small piece of plastic explosive, a detonator, and a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately, without close--and potentially dangerous--visual inspection, plastic explosives can be made virtually untraceable. In particular, detection of sheet explosives, typically having a thickness as small as one-quarter inch, has not been effectively accomplished by prior technologies.
The wide-scale attempts to fight the illegal drug trade indicates that narcotics detection is also extremely important. The need for a simple procedure for detecting drugs inside sealed containers, mail parcels, and other small packages, quickly and accurately, is immeasurable. Conventional detection methods are time-consuming, costly, and have marginal reliability.
Detection by means of NQR or NMR is possible for both explosives and narcotics, partially because they have as a constituent element .sup.14 N in crystalline form. Particularly with respect to narcotics, this is true of cocaine base, cocaine hydrochloride and heroine based narcotics. The hydrochloride forms of narcotics, such as cocaine hydrochloride, also contain quadrupolar nuclei .sup.35 Cl and .sup.37 Cl.
A significant factor in contraband detection by means of NQR in particular is that quadrupolar nuclei that are commonly present, and potentially readily observable, in narcotics and explosives include nitrogen (.sup.14 N) and chlorine (.sup.35 Cl and .sup.37 Cl), among possible other nuclei. Thus, in commercial applications, it is necessary to be able to detect quadrupolar nuclei contained within articles of mail, mail bags or airline baggage, including carry-on and checked luggage. While the resonant frequencies of the nitrogen in these substances differs for each chemical structure, these resonant frequencies are well defined and consistent. By applying an RF signal to a container having any of these suspected substances inside, and then detecting any quadrupolar resonance thus engendered by the application of RF pulses, the identity of the contraband substance can be easily determined.
NQR and NMR signals originate from the energy transitions associated with certain nuclei. To minimize noise and radio frequency power requirements and improve detection receiver sensitivity, conventional NQR and NMR systems use a narrow band, high Q, sample coil for both transmitting and receiving. There are, however, a number of factors that can significantly degrade the effectiveness of detecting NQR and NMR signals using this kind of narrow band, high Q, detection apparatus. Some of them are:
(1) the presence of large conductive materials inside the sample coil; PA1 (2) the presence of materials with high dielectric constant inside the sample coil; PA1 (3) temperature variations, which can significantly affect the value of the capacitance used for tuning and matching the RF coil antenna; PA1 (4) mechanical movement of the coil with respect to its surroundings; and PA1 (5) changes in size and shape of the coil and/or the shield caused by temperature changes or external baggage loading of the apparatus.
All of these factors can cause serious de-tuning of the detection apparatus, which in turn causes a sensitivity reduction in the detection sensitivity of NQR and NMR signals from the materials inside the sample coil.
Previously, for most applications of NQR and NMR, these conditions have not presented a serious drawback. The apparatus could usually be set up under near-optimum conditions, and the materials being investigated were usually well characterized. However, over the past few years several new applications have arisen which require NQR and NMR apparatus and methods for the detection of certain materials under adverse conditions (for instance, applications in which large volumes of largely unknown materials are under investigation).
Presently used NQR and NMR bulk substance detection systems have fundamental problems with sensitivity and accuracy. These techniques, which are branches of radio frequency spectroscopy, exploit fundamental properties of atomic nuclei. NQR exploits electrical properties of the atomic nuclei, and NMR correspondingly exploits the magnetic properties of the atomic nuclei. Other useful techniques involve nuclear precession, that generates an oscillating nuclear magnetic moment in the atomic nuclei. To observe an NQR or NMR signal from a sample, a radio frequency (RF) pulse is applied to the sample which is in phase with magnetic moments precessional frequency of the sample. This perturbation is momentarily applied to the sample to re-orient the spin of the nuclei in the sample. The nuclei which are not in equilibrium immediately attempt transition to an equilibrium state. As the nuclei return to the equilibrium state, a radio frequency (RF) signal known as the free induction decay (FID) occurs. The inventions RF-coil detects tis signal, which is subsequently amplified and processed. These RF pulses are applied as a pulse train or sequence that in turn generate a series nuclear induction signals that are correspondingly detected, processed and stored.
The sensitivity of an NQR/NMR detection RF coil relies on minimizing the time between the end of the RF pulse and the start of signal detection. The pulse sequence signal is detected between each RF pulse. The pulse spacing time, referred to as .tau., depends on the nuclear induction signals characteristic relaxation time and the detection receiver. The nuclear induction signal decays during the pulse train with a characteristic decay time T.sub.1p, that depends upon the electrical and magnetic environment of the nuclei of interest in .tau. time period. The shorter a .tau. period is, the longer a T.sub.1p ("spin-locking" time constant of the material). To improve the sensitivity of an NQR/NMR detection RF coil, Q-damping of the RF circuit is desirable after a high voltage RF pulse is generated, and subsequently removed as soon as the RF circuit ring down voltage has decayed below a predetermined level.
Prior teachings that deal with "de-Q'ing" of NMR receiver devices include U.S. Pat. No. 5,546,000 entitled "Method for the Reduction of Radiation Damping During Signal Acquisition in NMR Experiments," discloses a method of radiation damping during free induction decay in NMR measurements of samples having a narrow line width uses the active switching of the "Q" factor value of the coil circuit of an NMR detection probe. This teaching is intended for use at high operational frequencies, i.e. in typically very high Larmor frequency (greater than 100 MHz) and radiation damping. Furthermore, this teaching deals a received signals from samples that are so intense that the currents induced in the detection probe act like a low power transmit pulse. This requires a very high Larmor frequency of operation (substantially much greater than the observed NQR frequencies used in the invention herein), a sample volume substantially filled with the nuclide being sampled and a long spin-spin relaxation time T.sub.2. In the instant invention, the T.sub.2 relaxation time is much less due to the NQR detection system frequency regime of operation and radiation damping is not a problem since the relaxation dead time requires a long T.sub.2 time. This teaching does not recognize problems encountered of detecting a short T.sub.2 pulse with a long receiver ring-down dead time or problems associated with transients caused by the switching devices after a transmit pulse event for effective contraband detection.
Thus, there is need for hardware that can reduce dead time between the end of the detection head's transmit pulse and the beginning of the data acquisition thereof for efficient and effective specimen analysis while the detection receiver maintains a high-Q characteristic.